Nov
14
2009

Why is it easier to balance on a bicycle when in motion than when standing still?

Bicycle wheel.

Photo of bicycle wheel by Tom T. CC-BY.

If you didn’t know better, you might think that the act of balancing yourself on a bicycle would be extremely difficult — after all, how many other times in your life do you attempt to balance yourself on something only a 2 or 3 centimeters wide?

The reason it is easy to balance oneself on a moving bicycle is connected with the two wheels’ angular momentum. As the wheels turn faster, their momentum increases — and that makes it more difficult to change their direction.

This principle is tied in with Isaac Newton’s first law of motion. To simplify that law a bit, an object in motion will continue that motion unless there’s an external force to change it. So a turning wheel has a tendency to continue turning in the direction it’s turning unless there’s a force to make it change its direction. The force needed to make a wheel change direction increases as its speed increases. So if you’re balanced on a moving bicycle, it actually takes some force to knock it out of balance (such as by turning the steering wheel). If the rider were to disappear suddenly, the bicycle would stay balanced on its own until friction or other forces slow it down or change its direction. Similarly, with practice it is possible to get a coin or other disc to roll a long distance without further intervention.

The principle of bicycle balance is the same as that of a gyroscope. A gyroscope uses a turning wheel that maintains its position even when the framework around it moves. In the language of physics, the turning wheel of a gyroscope maintains its angular momentum. You might even think of the wheels on a bicycle as gyroscopes of sorts.

You can also see the principle at work with a child’s toy, the spinning top. As long as it’s spinning, once it is set in balance it will stay that way — until friction it slows down, reducing its angular momentum and thus its stability.

The real challenge in learning to use a bicycle, then, isn’t staying balanced, but learning how to balance yourself long enough to get the wheels turning rapidly. Once the wheels are rotating, it’s Newton’s law that’s keeping you balanced, not your own efforts.

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